created exch_info in api class

piker/brokers/deribit/api.py

@@ -344,6 +344,28 @@ class Client:
             'private/cancel', {'order_id': oid})
         return resp.result
 
+    async def exch_info(
+        self,
+        sym: str | None = None,
+
+        venue: MarketType | None = None,
+        expiry: str | None = None,
+
+    ) -> dict[str, Pair] | Pair:
+
+        pair_table: dict[str, Pair] = self._pairs
+
+        if (
+            sym
+            and (cached_pair := pair_table.get(sym))
+        ):
+            return cached_pair
+
+        if sym:
+            return pair_table[sym.lower()]
+        else:
+            return self._pairs
+
     async def symbol_info(
         self,
         instrument: Optional[str] = None,